Hi Mel,

On 02/05/2013 09:32 PM, Mel Gorman wrote:

On Tue, Feb 05, 2013 at 11:57:22AM +0000, Mel Gorman wrote:

quoted

quoted

quoted

+				migrate_pre_flag = 1;
+			}
+
+			if (!isolate_lru_page(pages[i])) {
+				inc_zone_page_state(pages[i], NR_ISOLATED_ANON +
+						 page_is_file_cache(pages[i]));
+				list_add_tail(&pages[i]->lru, &pagelist);
+			} else {
+				isolate_err = 1;
+				goto put_page;
+			}

isolate_lru_page() takes the LRU lock every time.

Credit to Michal Hocko for bringing this up but with the number of
other issues I missed that this is also broken with respect to huge page
handling. hugetlbfs pages will not be on the LRU so the isolation will mess
up and the migration has to be handled differently.  Ordinarily hugetlbfs
pages cannot be allocated from ZONE_MOVABLE but it is possible to configure
it to be allowed via /proc/sys/vm/hugepages_treat_as_movable. If this
encounters a hugetlbfs page, it'll just blow up.

I look into the migrate_huge_page() codes find that if we support the hugetlbfs
non movable migration, we have to invent another alloc_huge_page_node_nonmovable() 
or such allocate interface, which cost is large(exploding the codes and great impact
on current alloc_huge_page_node()) but gains little, I think that pinning hugepage
is a corner case. 

So can we skip over hugepage without migration but give some WARN_ON() info, is
it acceptable?

The other is that this almost certainly broken for transhuge page
handling. gup returns the head and tail pages and ordinarily this is ok

I can't find codes doing such things :(, could you please point me out?

because the caller only cares about the physical address. Migration will
also split a hugepage if it receives it but you are potentially adding
tail pages to a list here and then migrating them. The split of the first
page will get very confused. I'm not exactly sure what the result will be
but it won't be pretty.

Was THP enabled when this was tested? Was CONFIG_DEBUG_LIST enabled
during testing?

I checked my config file that both CONFIG options aboved are enabled. However it was 
only be tested by two services invoking io_setup(), it works fine..

thanks,
linfeng

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